React, updating state in child component using props after async call in parent component

Question

I'm building a weather app, and I'm looking for some advice for best practices for updating state in a child component based on props sent from parent component AFTER an async call from parent component.

I have a parent component that makes an async/await call in the componentDidMount() method to the navigator.geolocation and returns latitude and longitude which I want to send to the child component as props. Then, in the child component I need do an async/await call to the OpenWeatherMap API using the lat and long from the props. I then need to setState() using the response. I can't use componentDidMount() in the child because it mounts before the parent async/await call returns.

The problem is the application flow: The parent component mounts and renders, sending props to child as null. The child component mounts and renders with null props. Then, the async/await returns a response in parent, sets lat and long from response to state in the componentDidMount(), parent re-renders and sends props to child with correct values as lat and long. Child component updates with correct values in props. Now, at this point I need to setState() with those props, however I obviously can't do it in componentDidUpdate() without re-rendering into a infinite loop.

So, what's a good way to accomplish this task?

PARENT COMPONENT:

class Container extends React.Component {
  state = {
    cityState: {
      city: "",
      state: ""
    },
    latLong: {
      lat: null,
      long: null
    }
  };

  componentDidMount() {
    this.getCityAndState();
  }

  getCityAndState() {
    let latlng = "";
    if (navigator.geolocation) {
      navigator.geolocation.getCurrentPosition(async position => {
        latlng = `${position.coords.latitude},${position.coords.longitude}`;

        const response = await googleGeolocation.get(
          `json?latlng=${latlng}&location_type=APPROXIMATE&result_type=locality&key=${APIkey}`
        );

        this.setState({
          cityState: {
            city: response.data.results[0].address_components[0].long_name,
            state: response.data.results[0].address_components[2].short_name
          },
          latLong: {
            lat: position.coords.latitude,
            long: position.coords.longitude
          }
        });
      });
    }
  }

  render() {
    return (
      <div className="container">
        <LocationTime location={this.state.cityState} />
        <Forecast location={this.state.latLong} />
      </div>
    );
  }
}

CHILD COMPONENT:

class Forecast extends React.Component {
  state = {
    today: {},
    secondDay: {},
    thirdDay: {},
    fourthDay: {},
    fifthDay: {}
  };

  async componentDidUpdate() {
    ********** A bunch of logic in here to extract Weather API response 
into 5 separate arrays which will each be sent to the <Day /> child components
after setting the state to those arrays(which doesn't work in this 
life-cycle method, currently) **********

  }

  render() {
    return (
      <div className="forecast">
        <Day forecast={this.state.today} />
        <Day forecast={this.state.secondDay} />
        <Day forecast={this.state.thirdDay} />
        <Day forecast={this.state.fourthDay} />
        <Day forecast={this.state.fifthDay} />
      </div>
    );
  }
}

P.S. I always ask convoluted questions that end with fairly simple answers, lol

Answer 1

You may use the componentWillReceiveProps lifecycle method.

The first child component render some props like lat and lng are null, then you can do something like:

async componentWillReceiveProps(nextProps) {
  if ( this.props.lat !== nextProps.lat || this.props.lng !== nextProps.lng ) {
    const response = await YourAPICall(nextProps.lat, nextProps.lng)
    this.setState(/* set your things here */)
  }
}

Obviously this is just an outline...

Answer 2

Not sure why you use async/await instead of a normal fetch/axios call. In order to prevent entering in an infinite loop in your componentDidUpdate as you mentioned you need to run a conditional statement, something like:

componentDidUpdate(prevState){
 if (this.props.propertyYouWantToCheck !== prevState.propertyYouWantToCheck){
   // set your state/logic here
 }
}

Also you might want to consider to use fetch data only in the parent component and pass it down to the child component.